Abstract Internal combustion engines are well spread in traditional power-generation systems: given their reliability, low-cost and performance it is desirable to avoid their full substitution in the short term, but, rather, to rely on the reduction of their environmental impact. This can be achieved by implementing solutions that involve minor modifications on the devices, such as the use of different fuels. In this work, the fossil fuels replacement approach has been applied to a micro-cogeneration system based on a small water-cooled compression ignition engine. The effect of bioethanol introduction as a diesel substitute in six diesel-biodiesel blends has been studied in terms of performances of the whole energy-conversion system and of the exhaust gas emissions. Positive results have been obtained showing that with a small amount (3%) of bioethanol, enhancements can be fulfilled both on performances (with approximately an average 13%-boost of the electrical and thermal efficiency) and emissions (reaching nearly a 80% of smoke opacity reduction).